Organic compounds (organic molecules) are brought into a state having energy (excitation state) by absorbing light. By going through this excitation state, various reactions (photochemical reactions) are caused in some cases, while luminescence is generated in other cases; therefore, the organic compounds have found various applications.
As one example of the photochemical reactions, a reaction (oxygenation) between singlet oxygen and an unsaturated organic molecule is known (see Non-Patent Document 1, for example). Since a ground state of an oxygen molecule is a triplet state, the oxygen in singlet state (singlet oxygen) is not generated by a direct photoexcitation. However, under the presence of another triplet excitation molecule, the singlet oxygen is generated to achieve the oxygenation reaction. In this case, a compound capable of forming the triplet excitation molecule is referred to as a photosensitizer.
(Non-Patent Document 1) Haruo Inoue and three others, Basic Chemical Course Photochemistry I (Maruzen Co., Ltd.), 106–110
As described above, in order to generate the singlet oxygen, the photosensitizer capable of forming the triplet excitation molecule with the photoexcitation is necessary. However, since the ground state of ordinary organic compounds is a singlet state, the photoexcitation to the triplet excitation state results in a forbidden transition, and the triplet excitation molecule is hardly generated (a singlet excitation molecule is ordinarily generated). Therefore, as such photosensitizer, a compound which readily causes intersystem crossing from the singlet excitation state to the triplet excitation state (or a compound allowing the forbidden transition which the photoexcitation directly leads to the triplet excitation state) is in demand. In other words, such compound can be used as the photosensitizer and is useful.
Also, such compounds usually discharge phosphorescence. The phosphorescence is light emission caused by a transition between energies different in multiplicity and, in terms of ordinary organic compounds, means light emission caused when the triplet excitation state returns to the singlet ground state (in contrast, the light emission caused when the singlet excitation state returns to the singlet ground state is called fluorescence). One example of applicable fields of the compound capable of discharging the phosphorescence, i.e. the compound capable of converting the triplet excitation state into light emission (hereinafter referred to as phosphorescent compound), is an electroluminescence element using the organic compound as a luminescent compound.
The electroluminescence element is a device attracting attention as a next generation flat panel display element thanks to its characteristics such as thin and lightweight, high speed response, and direct current low voltage driving. Also, because of its self-luminous and wide viewing angle, the element has a comparatively good visibility and is considered to be effective as an element to be used as a display screen of mobile appliances.
In the case of using the organic compound as an illuminant, a light emission mechanism of the electroluminescence element is a carrier injection type. That is, by applying a voltage to an electroluminescence layer being sandwiched between electrodes, electrons injected form a cathode are recombined with holes injected from an anode in the electroluminescence layer to form excited molecules which discharge energy to emit light when they return to the ground state.
Types of the excited molecules can be, as is the case with the photoexcitation described above, the singlet excitation state (S*) and the triplet excitation state (T*). Further, a statistical generation ratio of the excited molecules in the electroluminescence element is considered to be S*:T*=1:3 (see Non-Patent Document 2, for example).
(Non-Patent Document 2) Tetsuo Tsutsui, Society of Applied Physics, Text for the Third Training Class of Organic Molecules/Bioelectronics Special-interest Group, 31–37 (1993)
However, the light emission (phosphorescence) from the triplet excitation state of general organic compounds is not observed at a room temperature, and, ordinarily, only the light emission from the singlet excitation state (fluorescence) is observed. This is because the ground state of the organic compounds is, in general, the singlet ground state (S0); therefore, T*-S0 transition (phosphorescence process) is a strong forbidden transition, and S*-S0 transition (fluorescence process) is an allowable transition.
Accordingly, a logical limit of internal quantum efficiency (a ratio of generated photons to injected carriers) in the electroluminescence element has been set to 25% based on the ratio of S*:T*=1:3.
But, since the T*-S0 transition (phosphorescence process) is allowed when the above-described phosphorescent compound is used, the internal quantum efficiency can logically be improved to 75 to 100%. That is, light emission efficiency of 3 to 4 times that of the conventional one can be achieved. Actually, electroluminescence elements using phosphorescent compounds have been proposed one after another, and high-level light emission efficiency thereof has been noted (see Non-Patent Documents 3 and 4, for example).
(Non-Patent Document 3) D. F. O'Brien and three others, Applied Physics Letters, vol. 74, No. 3, 442–444 (1999)
(Non-Patent Document 4) Tetsuo Tsutsui and eight others, Japanese Journal of Applied Physics, vol. 38, L1502–L1504 (1999)
A porphyrin complex whose central metal is platinum is used in Non-Patent Document 3, while an organometal complex whose central metal is iridium is used in Non-Patent Document 4, each of the complexes being the phosphorescent compound.
Further, by alternately stacking a layer containing the organometal complex whose central metal is iridium (hereinafter referred to as iridium complex) and a layer containing DCM2 which is a known fluorescent compound, it is possible to move triplet excitation energy generated by the iridium complex to DCM2 so as to cause the energy to contribute to light emission of DCM2 (see Non-Patent Document 5, for example). In this case, since the quantity of the singlet excitation state of DCM2 (25% or less under ordinary circumstances) is increased compared with the ordinary circumstances, the light emission efficiency of DCM2 increases. In other words, this means a sensitization effect of iridium complex which is a phosphorescent compound.
(Non-Patent Document 5) M. A. Baldo and two others, Nature (London), vol. 403, 750–753 (2000)
As proved by Non-Patent Documents 3 to 5, the electroluminescence element using phosphorescent compounds is capable of achieving the light emission efficiency higher than the conventional example (i.e. capable of achieving high luminosity with a small current). Therefore, it is considered that the electroluminescence element using phosphorescent compounds will assume a great importance in future developments as a measure for achieving high luminosity emission and high light emission efficiency.
The phosphorescent compounds are useful as the photosensitizer and also as a phosphorescent material for the electroluminescence element because they readily cause the intersystem crossing and readily produce light emission (phosphorescence) from the triplet excitation state as described above, and much hope is placed on the compounds; however, the number thereof is small under the present situation.
Among the small number of phosphorescent compounds, the iridium complex used in Non-Patent Documents 4 and 5 is called an orthometal complex which is a kind of the organometal complexes. Since this complex has a phosphorescence life of a several hundreds of nanoseconds and is high in phosphorescence quantum yield, a reduction in efficiency due to an increase in luminosity is smaller than that of the porphyrin complex, so that the complex is effectively used in the electroluminescence element. For this reason, such organometal complex is one of polestars for synthesizing a compound which readily causes the direct photoexcitation or the intersystem crossing to the triplet excitation state, or the phosphorescent compounds.
The iridium complex used in Non-Patent Documents 4 and 5 has a relatively simple ligand structure and exhibits a green light emission having a good color purity; however, it is necessary to change the ligand structure in order to change the light emission color to a different one. For example, in Non-Patent Document 6, various ligands and iridium complexes using the ligands are synthesized and several light emission colors are realized.
(Non-Patent Document 6) M. Thompson and ten others, Tenth International Workshop on Inorganic and Organic Electroluminescence (EL '00), 35–38
However, almost all of the ligands are limited to those capable of forming a five-membered ring with the central metal, and, under the current situation, a desired light emission color is selected among the ligands. That is to say, there is a problem that the number of usable ligands is still small.
Also, many of the ligands are difficult to synthesize or require a large number of steps for the synthesis, thereby leading to an increase in cost of the material itself. From the cost point of view, the yield of the organometal complex itself is also important.
Further, the organometal complex is generally subject to decomposition, and a decomposition temperature of the one which is less subject to decomposition is not high at all. Therefore, the poor heat resistance is problematic in the application thereof to an electronic device such as the electroluminescence element.
In view of the above, an organometal complex which is obtainable with a high yield by using a ligand which can be easily synthesized and excellent in heat resistance is in demand. By synthesizing such organometal complex, it is possible to obtain a photosensitizer and a phosphorescent material which are low in cost and high in heat resistance.
Therefore, in the present invention, it is intended to provide a novel organometal complex which is obtained with a high yield by using a ligand which can be easily synthesized. Particularly, it is intended to provide the novel organometal complex excellent in heat resistance.
Further, it is intended to provide an electroluminescence element which is high in light emission efficiency by manufacturing the electroluminescence element using the organometal complex. Further, it is intended to provide a light emission device which is low in power consumption by manufacturing the light emission device using the electroluminescence element.